OC_Connector_Sabre_Principal::searchPrincipals - return empty array instead of 0

OC_Connector_Sabre_Principal::searchPrincipals should return an empty array instead of 0, because sabredav uses the return value in foreach loop. see https://github.com/owncloud/3rdparty/blob/master/Sabre/DAVACL/AbstractPrincipalCollection.php#L146

return 0 will cause errors like https://github.com/owncloud/calendar/issues/96

please review @DeepDiver1975 @schiesbn @karlitschek